Activation patterns of rotator-cuff muscles from quantitative IVIM DWI after physical testing

Background The diagnostic value of clinical rotator cuff (RC) tests is controversial, with only sparse evidence available about their anatomical specificity. We prospectively assessed regional RC muscle activation patterns by means of intravoxel incoherent motion (IVIM) diffusion-weighted magnetic resonance imaging (MRI) after the execution of common clinical RC tests. Methods Ten healthy subjects (five males, five females) underwent three sessions of diffusion-weighted 3-T shoulder MRI before and after testing the supraspinatus (SSP, Jobe test, session 1), subscapularis (SSC, lift-off test, session 2, at least 1 week later), and infraspinatus muscle (ISP, external rotation test, session 3, another week later). IVIM parameters (perfusion fraction, f; pseudo-diffusion coefficient. D*; and their product, fD*) were measured in regions of interest placed in images of the SSP, SSC, ISP, and deltoid muscle. The Wilcoxon signed-rank test was used for group comparisons; p-values were adjusted using the Bonferroni correction. Results After all tests, fD* was significantly increased in the respective target muscles (SSP, SSC, or ISP; p ≤ 0.001). After SSP testing, an additional significant increase of fD* was observed in the deltoid, the SSC, and the ISP muscle (p < 0.001). After the SSC and ISP tests, no significant concomitant increase of any parameter was observed in the other RC muscles. Conclusion IVIM revealed varying activation patterns of RC muscles for different clinical RC tests. For SSP testing, coactivation of the deltoid and other RC muscles was observed, implying limited anatomical specificity, while the tests for the SSC and ISP specifically activated their respective target muscle. Relevance statement Following clinical RC tests, IVIM MRI revealed that SSP testing led to shoulder muscle coactivation, while the SSC and ISP tests specifically activated the target muscles. Key Points In this study, intravoxel incoherent motion MRI depicted muscle activation following clinical rotator cuff tests. After supraspinatus testing, coactivation of surrounding shoulder girdle muscles was observed. Subscapularis and infraspinatus tests exhibited isolated activation of their respective target muscles. Graphical Abstract

• After supraspinatus testing, coactivation of surrounding shoulder girdle muscles was observed.
• Subscapularis and infraspinatus tests exhibited isolated activation of their respective target muscles.Representative images of a 32-year old male participant: maps of the intravoxel incoherent motion parameters diffusion coefficient (D), perfusion fraction (f), perfusion-related pseudo-diffusion coefficient (D*), and blood-flow-related fD* before and after activation by the SSP test (Jobe test) are superimposed on transverse trace images

Background
Rotator cuff (RC) tears are a common health burden in the elderly population and can result in pain and limited function [1,2].To detect RC tears, an initial clinical examination is usually done to test for the weakness of the RC muscles [3].While a variety of corresponding tests exist, the most commonly used tests include the Jobe test for the supraspinatus muscle (SSP) [4,5], the lift-off test for the subscapularis muscle (SSC), and the external rotation test for the infraspinatus muscle (ISP) [6,7].
Because of the anatomic complexity of the shoulder, each muscle serves multiple functions which frequently overlap [8].Thus, some clinical RC tests are likely to not only activate the target muscle but also the surrounding muscles [9,10], which may result in limited diagnostic accuracy for RC tears, and might potentially influence the assessment of clinical outcomes after RC repair [11].Electromyography (EMG) studies have demonstrated that different examination positions can influence the extent of muscle coactivation in the RC other than the targeted muscle [9,[12][13][14][15].However, EMG reproducibility is considered to be limited, mostly due to the fact that it detects the activity of a small number of muscle fibers only [16].On the other hand, quantitative magnetic resonance imaging (MRI) offers noninvasive methods for characterizing exercise-induced regional microstructural changes in skeletal muscle [17].Among those is intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI) MRI, which describes the signal decay for increasing diffusion-sensitization (b-value) of the DWI sequence with a biexponential, i.e., two-compartment model, with three parameters: a diffusion coefficient D that is assumed to characterize the restricted and averaged thermal self-diffusion of extravascular tissue water; a pseudo-diffusion coefficient D*, assumed to reflect an averaged blood-flow in the network of capillary vessels (microcirculation) that are presumed to have an isotropic orientation distribution; and the perfusion fraction f, which characterizes the signal fraction contributed by the flowing capillary blood.Furthermore, the product fD* is thought to correlate with total microvascular blood-flow (perfusion) in the capillary network [18,19].Even though it has been demonstrated that IVIM can depict perfusion differences in the skeletal muscle after activation [20], there is little knowledge about activation patterns after physical testing of the SSP and SSC muscle [21,22], while no activation patterns have yet been described after physical testing of the ISP muscle.
Therefore, the aim of this study was to describe the regional RC muscle activation after performing common clinical tests of the SSP, SSC, and ISP muscle using IVIM imaging and to evaluate possible coactivation of muscles in the surrounding shoulder girdle beyond the targeted muscle.

Participants
The local institutional review board (Cantonal Ethics Commission Zurich, Switzerland) granted approval for this prospective study.Ten healthy volunteers, without a history of any known musculoskeletal pathology (including spine pathology, muscle injury, muscle disease) or prior shoulder surgery, were enrolled after providing written informed consent.Exclusion criteria included the presence of fever at the examination timepoint and incidental detection of any shoulder pathology during the baseline MRI.The research adhered to ethical standards set by the institutional and/or national ethics committee.Participants underwent a total of three examinations, each including a baseline MRI, a clinical shoulder test, and a post-activation MRI.The examinations took place between May and August 2023, with an interval of 1 week between each examination.Participants were acclimatized to the temperature of the MRI examination room, which was 21 °C.Female participants were examined only in the post-ovulatory phase to minimize the effect of sex-related inherent body temperature differences.

MRI protocol and clinical shoulder examination
MRI data were acquired on a 3-T system (MAGNETOM Prisma, Siemens Healthineers, Erlangen, Germany) using a dedicated 16-channel shoulder coil in the supine position.First, a transverse T1-weighted turbo spin echo sequence was acquired for anatomical reference.The subsequent IVIM acquisition employed a transverse fatsignal suppressing monopolar-pulsed single-shot echoplanar diffusion-weighted sequence that acquired data at multiple b-values (0, 20, 40, 60, 80, 100, 200, 300, 400, 500, 600, 700, 800 s/mm 2 ) in three orthogonal diffusionencoding directions, from which the trace was computed.The sequence parameters of the protocol are specified in Table 1.
After the acquisition of MRI sequences at rest, participants underwent muscle activation by clinical shoulder examination in the MRI scanner room.For the SSP, the arm was abducted to 90°, then angled forward 30°, and internally rotated with the thumb pointing to the floor.The participant then resisted the examiner's downward pressure on his/her arm (Jobe test) [4].For the SSC, the participant was instructed to move the hand around the back to the lumbar region with the palm facing outward and attempted to lift his hand off his back against the examiner's resistance (lift-off test) [7].For the ISP, the participant's elbow was flexed 90°and then internally rotated 45°with a towel roll placed under the participant's armpit.The participant was then instructed to externally rotate the arm against resisted pressure from the examiner while holding the towel in place (external rotation test) [23].The examinations were conducted with the application of maximum tolerable resisted pressure for two minutes while keeping the participant's arm in position, as previously described by Nguyen et al [22].After the clinical shoulder examination, the participants immediately underwent a second MRI scan.

IVIM image processing
The biexponential signal equation model was assumed for the normalized decay of the diffusion-weighted signal as a function of b-value: in which S 0 is equilibrium magnetization at b = 0 s/mm 2 [24].Maps of IVIM parameters were created voxel-wise in the regions of interest (ROIs) and additionally, IVIM parameters were estimated from magnitude averages over all voxels in the respective ROIs by fitting the signal to Equation [1] in two steps using a custom-written Matlab script (version R2023a, Mathworks, Natick, MA) that invoked Matlab's built-in Trust-Region-Reflective nonlinear least-square fitting algorithm ("lsqnonlin").In the first step, a mono-exponential function was fitted to the signal-decay curve for b-values of 200 s/mm 2 and larger, to obtain parametric maps of D and of the y-intercept S 0,high .In a second step, the Equation [1] was fitted to the signal-decay curve, including data acquired at all b-values, searching for the best values for f, D*, and S 0 while keeping D and S 0,high = S 0 (1 − f ) at constant values obtained from the first step.

Quantitative image analysis
ROIs were manually drawn over the b = 0 s/mm 2 images on the slice in which the maximum cross-sectional area of the respective muscle of interest was visible (SSP, deltoid, SSC, ISP), attempting to exclude tendons and fasciae.In order to assess the activation of the tested muscles, absolute intramuscular changes in IVIM parameters (D, f, D*, fD*) were evaluated.The selectivity of these changes was assessed using the mean of relative intermuscular changes.

Statistical analysis
SPSS Statistics (version 29, IBM, Armonk, NY, USA) was used for all statistical analyses.The validity of assuming a normal distribution was examined with the Shapiro-Wilk test and the homogeneity of variances was evaluated with Levene's test.Data are given as mean ± standard deviation for normally distributed data, or as median with interquartile range in parentheses.The Wilcoxon signed-rank test was performed to evaluate whether quantitative parameters differed before and after exercise-induced activation for each muscle; p-values were adjusted for multiple testing using the Bonferroni method; p-values ≤ 0.0125 were considered significant.Intraclass correlation coefficients for absolute agreement were calculated using a two-way mixed model to assess the reproducibility of IVIM parameters at rest between the three different MRI sessions; p-values below 0.05 were considered significant.

Participants
The age of the ten study participants (five females and five males) was 32.0 ± 4.7 years (mean ± standard deviation).The body mass index was 23.0 ± 2.6 kg/m 2 .None of the volunteers were excluded during the study interval due to signs of fever, and no pathologies or incidental findings were identified on the anatomical MRI sequences.

Quantitative image analysis
Baseline    3), whereas there were no significant changes in the other muscles (deltoid, SSP, and ISP).For the ISP test, a similar activation pattern was observed: whereas all IVIM parameters in the ISP were significantly larger after activation (D: 4.1%, f: 108.5%, D*: 58.4%, fD*: 229.2%;Table 4), the changes in the other muscles (deltoid, SSP, and SSC) were non-significant.

Discussion
This study aimed to measure regional changes of IVIM parameters in the RC muscles induced by three different physical RC muscle tests to explore if the muscles of the surrounding shoulder girdle muscles were coactivated after the respective clinical test.
The first main finding was that the SSP test did not selectively activate the SSP muscle, as coactivation of the deltoid, SSC, and ISP muscles was observed.
It is well known that physical activity causes a global redistribution of blood-flow in favor of the active muscle group [25].IVIM has been successfully used to investigate changes in muscle perfusion following activation, demonstrating the capability to identify the specific muscle involved in a particular task [20].In the present study, IVIM parameters observed at rest and after activation were largely consistent with the findings of Nguyen et al [21,22], which may highlight the reproducibility of IVIM for the assessment of muscle activation in the shoulder girdle.Differences between the aforementioned and the present study could be explained by variations in imaging protocol parameters, as IVIM parameters are heavily dependent on tissue relaxation properties [26].
Our results are also in agreement with EMG studies that report coactivation of these muscles in the Jobe position when testing the SSP [12][13][14] and are supported by the knowledge that during scapular plane abduction, the deltoid has been identified as the principal movement driver, while the SSC and ISP are activated to stabilize the humeral head against the glenoid, providing a fulcrum for the deltoid in the early phases of abduction [13,27].
Numerous previous studies have found that SSP testing for pathologies has an unsatisfactory diagnostic accuracy: in a review by Longo et al, the Jobe test was found to have a sensitivity of less than 80% in four of six studies and a specificity of less than 80% in five of six studies [28].On the other hand, the anatomical validity of the lift-off test for SSC testing has been evaluated and confirmed [15,29], while the improved diagnostic performance might be attributed to the fact that the lift-off test isolates the SSC from the internal rotating and adducting forces of the pectoralis major [30].In contrast to our study, a coactivation the posterior deltoid was observed during the lift-off test in a study by Nguyen et al [22].The authors hypothesized that this may be caused by the requirement for some degree of shoulder extension in the lift-off position.These differences may be caused by the fact that this study evaluated the ROI average of IVIM parameters for the entire deltoid muscle rather than separately analyzing distinct muscle regions.
In the present study, only the ISP was activated during the seated external rotation test, while no activation of the deltoid or other RC muscles was recorded.Although there are conflicting results in the current literature regarding RC coactivation during external rotation for ISP testing [8,31], the seated position is preferred over the supine position to minimize middle and posterior deltoid muscle activity [32], which is in line with our results and may be due to the lower capsular strain and muscle tension in the seated position compared to other testing positions [31,32].Moreover, a towel roll was used in our study, which is supposed to ensure a proper technique for the participant to minimize shoulder abduction while externally rotating the shoulder [31].
This study has several limitations.First, the sample size of this investigation was small, which limits the generalizability of our findings.Second, a delay between test execution and MRI acquisition was inevitable, even though it was minimized by performing the tests in the MRI examination room.Third, only healthy volunteers were evaluated in this study.
In conclusion, this study used IVIM to demonstrate selective muscle activation in the RC after performing different clinical shoulder examinations.An isolated activity increase of the SSC and ISP was observed after a lift-off and external rotation test, while for the SSP, scattered activation patterns of the surrounding RC muscles after the Jobe test were found.Our results suggest limited anatomical specificity for the Jobe test to target the SSP, while tests for the SSC and ISP showed improved isolation of the target muscle.Our results aligned with results from prior studies that studied activation-related changes of the RC after physical testing, which corroborates the reproducibility of the IVIM technique for functional imaging of the RC muscles.As a future perspective, IVIM may be useful for a comprehensive assessment of how patient positioning affects RC muscle activation, with the advantage of being noninvasive and providing spatial information on the entire RC rather than a small number of fibers as in EMG.This may facilitate the design of more effective clinical RC tests or the identification of activation patterns that may influence the outcome after RC repair.

I
IVIM DWI r revealed varying activation patterns of r rotator--c cuff m muscles for different clinical t tests A Activation patterns of rotator--c cuff muscles f from quantitative IVIM DWI after physical testing E Eur R Radiol E Exp (2024) Marth AA, Spinner GR, von D Deuster C C, Sommer S, Sutter R R, N Nanz D D; DOI: 10.1186/s41747--0 024--0 00487--5 5

Fig. 1
Fig. 1 Representative images of a 32-year-old male participant: maps of the intravoxel incoherent motion parameters diffusion coefficient (D), perfusion fraction (f), perfusion-related pseudo-diffusion coefficient (D*), and blood-flow-related fD* before and after activation by the SSP test (Jobe test) are superimposed on transverse trace images.A significant increase of D, f, D*, and fD* was observed.Delt, Deltoid muscle; ISP, Infraspinatus muscle; SSC, Subscapularis muscle; SSP, Supraspinatus muscle

Fig. 4
Fig. 4 ISP test (external rotation test): signal decay averaged in the muscle region of interest and across all participants before the test (blue curve) and after the test (red curve).Error bars represent the standard deviation across subjects per b-value.Delt, Deltoid muscle; ISP, Infraspinatus muscle; SSC, Subscapularis muscle; SSP, Supraspinatus muscle

Fig. 3
Fig. 3 SSC test (lift-off test): signal decay averaged in the muscle region of interest and across all participants before the test (blue curve) and after the test (red curve).Error bars represent the standard deviation across subjects per b-value.Delt, Deltoid muscle; ISP, Infraspinatus muscle; SSC, Subscapularis muscle; SSP, Supraspinatus muscle

Table 1
Sequence acquisition parameters Both sequences were acquired in transverse orientation

Table 2
Diffusion coefficient D, perfusion fraction f, pseudo-diffusion coefficient D*, and blood-flow-related fD*, before and after performing the supraspinatus test Data are given as median with interquartile range in parentheses.The p-values were obtained by applying the Wilcoxon signed-rank test with Bonferroni correction for multiple testing; p-values ≤ 0.0125 were considered significant ISP Infraspinatus muscle, SSC Subscapularis muscle, SSP Supraspinatus muscle

Table 4
Diffusion coefficient D, perfusion fraction pseudo-diffusion coefficient D*, and blood-flow-related fD* before and after performing the infraspinatus test Data are given as median with interquartile range in parentheses.The p-values were obtained by applying the Wilcoxon signed-rank test with Bonferroni correction for multiple testing; p-values ≤ 0.0125 were considered significant ISP Infraspinatus muscle, SSC Subscapularis muscle, SSP Supraspinatus muscle

Table 3
Diffusion coefficient D, perfusion fraction f, pseudo-diffusion coefficient D*, and blood-flow-related fD* before and after performing the subscapularis test Data are given as median with interquartile range in parentheses.The p-values were obtained by applying the Wilcoxon signed-rank test with Bonferroni correction for multiple testing; p-values ≤ 0.0125 were considered significant ISP Infraspinatus muscle, SSC Subscapularis muscle, SSP Supraspinatus muscle